PROJECT SUMMARY In many tissue types, cell fate and consequent tissue organization are dictated by the orientation of the mitotic spindle with respect to the cell boundaries. During such processes as organismal development and tissue homeostasis, spindle orientation dictates the plane of cell division, and thus whether a cell divides symmetrically or asymmetrically. Symmetric stem cell divisions result in two identical stem cells, whereas a switch to asymmetric division results in one stem cell and a differentiated cell. Thus, proper coordination of spindle position with the particular needs of a tissue or cell type is critical during numerous biological processes. Improper spindle orientation can compromise asymmetric stem cell divisions, impair differentiation, and lead to defects in tissue development and homeostasis. In fact, unchecked symmetric and asymmetric divisions have both been directly linked to cancer initiation and progression. A key effector of spindle orientation is the molecular motor cytoplasmic dynein. This motor is anchored at the cell cortex from where it orients the spindle through precisely tuned interactions with microtubules. It is unclear how cortically anchored dynein motors perform this function with appropriate directional and temporal control to achieve proper tissue specific functions. The lack of such information presents an impediment towards the development of effective therapies that may prevent or reverse defects in tissue organization that can lead to developmental disorders or cancer. In the proposed studies, we will use the simple model organism budding yeast ? in which dynein and many of its regulators are highly conserved ? and a combination of in vivo, in vitro, and biophysical methods to determine the mechanisms by which dynein is activated to perform its spindle orientation function, and regulated to achieve appropriate directionally biased spindle movements. Our specific aims are: (1) determine how cortical dynein activity is switched on, and (2) determine how dynein-mediated spindle movements are directionally biased.